Current methods for producing drill bits/coreheads (coring bits) utilise a matrix or a steel body.
In the matrix type, tungsten carbide powder matrix is formed in a thick shell around a steel inner core which carries the threaded connection. The cutters are then brazed on to the pre-formed matrix shell.
This method is suitable since the tungsten carbide matrix is very resistant to fluid erosion and abrasive wear, natural diamonds can be included in the matrix shell for gauge protection, and relatively complex shapes can be produced.
However, the method suffers from the disadvantages that possible breakdown of bond between the matrix shell and steel core may occur, manufacture of the graphite mould is precision work requiring high labour input, and the cost is high due to the quantity of carbide required.
Also, the differential of contraction between matrix shell or steel core may cause cracking especially in the larger products and further, poor quality of the matrix body formed necessitates extensive hand fettling.
In the steel body type, the normal method of manufacture is by machining from the solid using multi-axis milling machines and then hard-facing using welding or spray metal techniques prior to the installation of the cutters. These cutters are either brazed in place or pressed into prepared holes and held in place by an interference fit.
The advantages of the steel body type are a single unit construction with no possibility of break-up due to bond failure or cracking, low cost materials, and CNC multi-axis milling machine techniques give good repeatability for batch production. However, the steel body type method is labour intensive, in that hard facing has to be applied after machining, and any surplus hardfacing has to be hand-ground away from cutter pockets prior to installation. Also, the allowable complexity of shape is restricted by limitations of machining capabilities.
It has previously not been considered a viable solution to manufacture drill bits/coreheads (coring bits) utilising investment casting techniques; the matrix and CNC machining approach being far more established and understood than this hitherto unkown method of manufacturing.
The accepted standard method of manufacturing an investment casting for industrial products such as aircraft turbine blades and engine components is as follows:
A master mould be manufactured to cast accurate wax males of the product required. The wax males are then coated with a ceramic material by dipping them in a slurry and then raising sand on the wet slurry. This is done a number of times, allowing the slurry and sand coating to dry before re-dipping.
In this way, a thick coating of material is built up around the wax male. The coated wax male is then furnaced to bake the coating and melt out the wax, thus creating an accurate ceramic mould of the product to be cast.
Under normal circumstances, this method of manufacture would not be used to produce a steel-bodied drill bit or corehead (coring bit) due to the fact that it would require subsequent hard facing after casting in order to withstand the fluid erosion and abrasive wear experienced downhole. The application of this hard facing by spray metal or welding techniques would cover or damage the accurately-formed profile of the investment cast product thus spoiling the dimensional accuracy and therefore defeating the purpose of using this process in the first place.
It is an object of the invention to obviate or mitigate the above disadvantages by utilising the investment casting process in a novel method of manufacture to produce a highly accurate and, if required, complex casting, which needs little refinishing prior to installation of the cutters.